1. Field of the Invention
The present invention generally relates to a head driving unit and an image forming apparatus, and more particularly, to an image forming apparatus having a liquid drop discharging head and a head driving unit for driving the liquid drop discharging head.
2. Description of the Related Art
An ink jet recording apparatus to be used as image forming apparatuses such as a printer, a facsimile, a photocopier, a plotter (an imaging apparatus) is provided with an ink jet head for discharging liquid drops consisting of a nozzle that discharges ink drops, an ink flow path (referred to as a discharging room, a pressure force room, a pressure liquid room, or a liquid room) that is connected to the nozzle, a driving unit that pressurizes the ink in this ink flow path.
Liquid drop discharging heads such as a liquid drop discharging head for discharging a liquid resist and for discharging a sample of DNA as liquid drops are known, but we focus on ink jet heads in the following description.
At least three types of ink jet heads are known in the art.
The so-called piezoelectric type uses a piezoelectric element as a driving unit to pressurize ink in the ink flow path. The piezoelectric element moves a diaphragm forming a wall surface of the ink flow path and changes the volume of the ink flow path to discharge ink drops.
The so-called bubble type uses a heat generating resistor. The heat generating resistor heats the ink in the ink flow path and generates bubbles to pressurize the ink.
On the other hand, an electrostatic type (cf. Japanese Patent Laid-Open Application No. 6-71882) uses a diaphragm and an electrode facing each other forming a wall surface of an ink flow path. The diaphragm is moved by the electrostatic force generated between the diaphragm and the electrode and changes the volume of the ink flow path to discharge ink drops.
A driving unit of an ink jet head generates a common driving waveform to be applied to a driver corresponding to each nozzle and controls a switch corresponding to each driver depending on a recording image.
The driving unit drives the head by switching on and off the common driving waveform applied to each driver in response to a signal indicating the recording image.
By the way, in order to obtain a suitable driving waveform to be applied to each driver element of an ink jet head, driving parameters such as driving voltage, pulse width, rise time and fall time of a pulse must be set at an optimum condition depending on the ink jet head.
Therefore, in the case of a conventional head driving unit using a common driving waveform, whenever the recording density is changed, for example, the common driving waveform, that is, the driving parameters defining the common driving waveform, must be changed as well in order to obtain the optimum dot diameter most suitable for the recording density.
When multiple value recording is required, the quantity of ink in an ink drop discharged must be changed, nozzle by nozzle, to change dot diameters. Different driving waveforms must be applied to each nozzle. The more head nozzles required, the more driving waveform generators are required to generate common driving waveforms. The cost increase is not negligible.